1. Field of Invention
This invention relates generally to apparatus used for drilling branch wells from a main well, and more specifically to apparatus for drilling lateral wells from cased wells, for the purpose of producing oil and gas from subsurface formations.
2. Description of the Prior Art
Conventional technology provides for the drilling of a well from the surface to a predetermined depth beneath the surface into a subterranean formation containing hydrocarbon reserves. Most conventional wells have traditionally been substantially vertical. However, current technology now provides for the drilling of deviated or non-vertical wells using directional drilling technology.
Directional drilling technology also allows for secondary, branch or lateral wells to be drilled laterally from the primary or main well. A primary well including more than one secondary or lateral well is typicaly referred to as a multilatal well. Lateral wells are often drilled and produced through a port in the casing of the primary well. The port typically comprises a window pre-cut or pre-formed in a section of the steel casing string. The casing window system is available in various oilfield tubular material grades. Typically, the completed casing window is overwrapped with composite materials, such as fiberglass.
U.S. Pat. No. 4,415,205, issued on Nov. 15, 1983, to Rehm et al, discloses a method and apparatus to complete triple branch wells using separate drill and casing templates. In Rehm et al a special casing is used having a series of vertically separated windows, or easily penetrable exit ports, corresponding to the desired location of branch wells. The windows are filled with fiberglass.
U.S. Pat. No. 5,458,209, issued on Oct. 17, 1995, to Hayes et al, discloses a method and system for drilling lateral wells with respect to a main well which is cased by a casing string. In Hayes et al there is disclosed the use of a pre-cut opening or window in the steel casing, covered by fiberglass, which can be easily drilled.
Use of the prior art systems of Rehm et al and Hayes et al, in which a per-cut or pre-formed opening is filled or covered with an easily drillable material such as fiberglass, has proven to be less than completely satisfactory. The fiberglass material simply cannot withstand the high pressures fequently encountered in oil and gas wells, sometimes being in excess of 10,000 psi. In addition, the exposed fiberglass can be damaged easily when installing the casing string in a well.
In U.S. Pat. No. 5,615,740, issued Apr. 1, 1997, to Comeau et al and assigned to the assignee of the present invention, there is disclosed a system for use in high pressure environments typical in oil and gas drilling. Comeau et al utilize a pre-cut window in the casing which is covered with easily drillable material, such as fiberglass. In addition, a retrievable pressure sleeve is fixed within due interior of the casing, adjacent the window in the casing. The sleeve is pressure sealed to the interior of the casing and the window space between the sleeve and the fiberglass wrap filled with fluid to provide protection from pressure damage to the fiberglass window covering. Once the casing has been cemented in place, the sleeve can be retrieved to the surface and drilling through the window can commence.
When a typical window opening in the casing is sheathed with a composite material, such as fiberglass, as thought in the prior art, the torque strength of the resulting composite is low for application in a high torque environment often encountered when installing a casing string into a well. A high torque as referred to herein can be considered to be torque greater than approximately 10,000 foot-pounds. The addition of the internal sleeve, as thought by Comeau et al, does not add significant strength for applications requiring high torque. Also, the internal sleeve presents the requirement for an additional trip into the well so that the internal sleeve can be removed to the surface before drilling out the window.
As a result, there remains a need in the industry for an improved apparatus and method for drilling lateral wells in high pressure environments using a casing having an easily drillable exit port and having the capability to withstand torque.